In hard disk drives, data is written to and read from magnetic recording media, herein called disks, utilizing magnetoresistive transducers commonly referred to as MR heads. Typically, one or more disks having a thin film of magnetic material coated thereon are rotatably mounted on a spindle. An MR head mounted on an actuator arm is positioned in close proximity to the disk surface to write data to and read data from the disk surface.
During operation of the disk drive, the actuator arm moves the MR head to the desired radial position on the surface of the rotating disk where the MR head electromagnetically writes data to the disk and senses magnetic field signal changes to read data from the disk. Usually, the MR head is integrally mounted in a carrier or support referred to as a slider. The slider generally serves to mechanically support the MR head and any electrical connections between the MR head and the disk drive. The slider is aerodynamically shaped, which allows it to fly over and maintain a uniform distance from the surface of the rotating disk.
Typically, an MR head includes an MR read element to read recorded data from the disk and an inductive write element to write the data to the disk. The read element includes a thin layer of MR or GMR sensor stripe sandwiched between two magnetic shields that are electrically connected together but are otherwise isolated. A constant current is passed through the sensor stripe, and the resistance of the magnetoresistive stripe varies in response to a previously recorded magnetic pattern on the disk. In this way, a corresponding varying voltage is detected across the sensor stripe. The magnetic shields help the sensor stripe to focus on a narrow region of the magnetic medium, hence improving the spatial resolution of the read head.
The space between the shields and the magnetoresistive sensor strip are called the read gaps. Because the shields are electrically conductive, it is desirable to prevent the sensing current from leaking into the shields. For this reason, a thin dielectric film may be used to fill the read gaps so as to insulate the sensor stripe from each shield. Each of the read gaps in current devices may be in the range of 200 Angstroms (20 nanometers), while next generation devices may have read gaps in the range of 100 Angstroms.
The read/write heads are initially produced in the form of a wafer containing approximately 10,000 or more heads. A finished wafer is then cut into rows and subsequently diced into individual read/write heads in a configuration known as a slider. The surface of a read/write head that faces the disk surface is known as the air-bearing surface (ABS). The ABS portion of each head is created when the wafer is cut into rows. This row of heads, with the ABS of each head exposed, is then polished to achieve a precise stripe height for the MR element. This polishing step, also known as lapping, removes a great deal of the material of the MR head to create the slider. For example, 3,000 nanometers of original material may be lapped back to create a stripe height of 120 nanometers.
While it is known to measure the electrical isolation between the MR head and the surrounding shields during the manufacture of the wafer, prior to the present invention it is not believed to have been possible to directly measure the electrical isolation of the MR head from the shields during the lapping process or subsequent thereto, such as once a slider has been produced. Unfortunately, due to the large amount of material removed from the row of heads during the lapping process, a great deal of excess material is present and a certain portion of the sliders may be produced with very tiny pieces of material bridging the gaps between the MR head and the shields. These pieces of material are generically described as smears. The smears may take the form of micro shorts, two-sided shorts, single-sided shorts, or other types of shorts. Micro shorts are so small as to be incapable of carrying much current and are known to contribute two-state noise to the output of the MR read head.
Two-sided shorts are a pair of shorts that exist on opposite sides of the read element, so that the sensing current is effectively shunted, at least in part, around the magnetoresistive sensor of the read element and through the shield, thus reducing the output level of the read head. While it is theoretically possible to detect two-sided shorts by measuring the resistance of the MR head, it is not practical to do so because there is typically a large tolerance in the allowable resistance of the MR heads, such as between the range of 30–80 ohms. A range this broad is used because of the array processing of the MR heads as described above, wherein an entire row of heads are lapped simultaneously. Because of this broad range, parallel shunt paths of low resistance from two-sided shorts are difficult to detect.
Single-sided shorts exist on only a single side of the MR sensor and are particularly difficult to detect. They can provide a path to ground for charges that may build up on the shields. Unfortunately, this path to ground sometimes passes through the GMR element and the preamplifier stage of the disk drive electronics, and may damage the sensor or its associated circuitry. This is especially true for single-ended preamps that ground one side of the sensor.
It has been discovered (through manufacturing audits and failure analysis using scanning electron microscopy (SEM)) that some percentage of sliders may have at least one type of these smear defects and that they will at least partially degrade performance in the assembled disk drive. This degraded performance can appear as head degradation such as an increased bit error rate in the performance of the disk drive, a lower signal output, or an increased likelihood of damage from an electrostatic discharge event. Because each new generation of disk drives typically has smaller dimensions, particularly the gaps between the MR element and the shields, this problem will only continue to increase in subsequent generations of disk drives. Furthermore, even if it were desired to add further testing capability to a slider, there is a severe shortage of space on the exterior of the slider to add additional electrical contact pads in addition to the existing pair of pads for the read element and the existing pair of pads for the write element.
While at one time in the manufacture of MR heads for disk drives, the wafer level defects were the dominant source of manufacturing defects, as processes have been improved the reduction of defects resulting from the above-described smears have taken on increased importance. It is against this background and a desire to improve on the prior art that the present invention has been developed.